1. Field
The present invention relates to support structures and methods for handling thin sheets during inspection, and particularly for handling thin transparent sheets during transmission-mode measurements
2. Technical Background
Recently, significant attention has been focused on the detection of defects in transparent substrates, for example glass sheets, primarily due to the popularity and acceptance of the liquid crystal display (LCD) television. As such, the industry is now challenged with meeting increased volume demands while delivering substrates that comply with stringent LCD transmission-mode specifications. Additionally, the transparent sheets used in the LCD industry have been growing in size while, at the same time, remaining the same thickness or even becoming thinner. Accordingly, it is a challenge to hold a large, thin, transparent sheet securely so that inspection measurements may be made, while at the same time maintaining measurement accuracy in light of errors induced by the structure used to hold the transparent sheet.
Transmission mode measurements include passing light through the transparent sheet from one planar face to the opposite planar face, and measuring how the light changes as it passes through. Transmission mode measurements may be used to detect process-induced features like inclusions, thickness variations, cord, streak, and stress, for example, in the transparent sheet.
Taking stress measurements for example, the manner in which the sheet is held will affect the measurement because stress may be induced by the interaction of the holding structure and the shape of the sheet. If a sheet were completely flat, its shape would not change when held in a substantially planar configuration by a holding structure and, accordingly, the holding structure would not affect the stress measurement. However, transparent sheets are not completely flat; instead, they include some (albeit small in most instances in the display industry) shape variance as, for example, warp, bow, convexity, or concavity, which is not likely to be the same from sheet to sheet. Further, such shape variance may be included in varying degrees in different areas of the sheet. Accordingly, when a transparent sheet is flattened by a holding structure during measurement, the sheet shape changes, which introduces errors in transmission-mode measurement. For example, the transparent sheet, in its nominal resting state, includes a stress distribution which affects light transmittance through the sheet. When the sheet is flattened, as in a support structure, the stress distribution changes so as also to include stress induced by the support structure when flattening the sheet. Thus, accuracy of the sheet stress measurement is affected. It is desirable to account for and remove the inaccuracy induced by the support structure to the extent possible. However, it is not always easy to differentiate what stress is induced by the support structure, and what stress is naturally occurring in the transparent sheet.
In light of the above, there is a need for an inspection method and apparatus which can hold large, thin, transparent substrates securely, allow support-structure-induced error easily to be detected accounted for and removed to the extent possible.